The present invention relates generally to systems and methods for detecting light, and more particularly to systems and methods for large dynamic range light detection.
It is often desirable to detect (and possibly quantify) light emitted from light emitting samples or substances. Such detection capability is useful in various assays, such as, for example, DNA sequencing assays where different fluorescent substances may be attached to different amino acids. For certain assays it is also desirable to quantify the light emitted by a sample. However, light emitted by a light emitting sample, such as a sample containing a fluorescent substance, may be in a 6 decade (106) range of intensity for a given stimulation intensity. Samples may be distributed over an area that is stimulated by a stationary or scanned point light source of a single color, with the emitted light directed to a point optical detector, and the detector's electrical output quantized to a digital value representing the optical power detected in a given time interval. The set of values may be organized into a 2-dimensional array representing fluorescent emission over the sample area, where a given sample detection time corresponds to a given point in the sample area. Alternatively, the data may be collected serially, in time, and arranged in any way that correlates a particular sample with a particular reading.
In certain applications requiring an optical detection power range of roughly 0.3 pW to 300 nW, for example, and the requirement of a signal-to-noise ratio of about 10 at 50 pW in a time interval of 5 μs, a PMT (photomultiplier tube) detector must be used. The PMT's practical dynamic range for this time interval is between 3 and 4 decades, limited at the low end by signal-to-noise and at the high end by PMT anode current rating. The PMT gain may be adjusted for 300 nW at maximum anode current but 0.3 pW will not be detected, or for usable signal-to-noise at 0.3 pW but 300 nW will be above the PMT anode current rating. In prior art implementations, multiple scans with different stimulation intensities or PMT responsivities were required to be carried out for 6 decade detection, consuming extra time and possibly degrading the sample due to increased light exposure.
Therefore it is desirable to provide systems and methods that overcome the above and other problems.